Kathir bin Murrah narrated from Sayyiduna Utbah bin Abd (may Allah be pleased with him), who said: The Messenger of Allah (peace and blessings be upon him) said on the day of Banu Qurayzah and Banu al-Nadir: "Whoever sends an arrow into this fortress, Paradise has become obligatory for him." Utbah (may Allah be pleased with him) said: I sent three arrows in.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14371